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Help! Tach Problem

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
Hey everyone,

The tach has never worked in my car. I would start it, and the needle would just climb all the way up and stay there.

I took the dash apart, removed the tach, and took a volt meter to the terminals where the tach plugs in.

I have ground, I have 12V, and I have a signal that fluxuates as I rev the engine (I assume this is the signal from the tach). This made me assume that the problem was the tach circuit.

I put in a new circuit and a new filter from Mid American Motorworks. Same problem. Could there be a problem with the gauge itself?

PLEASE HELP!
 

wishuwerehere82

Well-known member
Joined
Sep 6, 2003
Messages
2,314
Location
Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe
You might want to check the connector at the back of the gauge cluster. Mine was half unplugged once and all the gauges went to full scale or zero randomly. Im pretty sure the tach signal goes through that harness.
Just remove the glove box liner and reach behind the cluster to make sure the connector is plugged in all the way til it clicks.
There is a filter capacitor on the tach that averages the pulses that you see coming into the tach to a DC voltage so that the tach stays steady, which could go bad.
So it might be a bad tach gauge itself, or the capacitor.
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
Ok, update.

Turns out the connection to the tachometer itself was bad. I managed to fix it by cleaning it and adding some aluminum washers to increase the amount of surface area that was coming into contact with the circuit board.

Now the problem is that the gauge is not even close to calibrated.

I can zero the gauge, but it shows my idle speed as about 1400 rpms.

Is there any way to calibrate the gauge, or do I just have to get a different circuit?

Thanks!!!
 

wishuwerehere82

Well-known member
Joined
Sep 6, 2003
Messages
2,314
Location
Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe
Ok, update.

Turns out the connection to the tachometer itself was bad. I managed to fix it by cleaning it and adding some aluminum washers to increase the amount of surface area that was coming into contact with the circuit board.

Now the problem is that the gauge is not even close to calibrated.

I can zero the gauge, but it shows my idle speed as about 1400 rpms.

Is there any way to calibrate the gauge, or do I just have to get a different circuit?

Thanks!!!
No calibration. But your contacts need to be good for it to read accurate. I think aluminum washers need to be replaced with copper ones to get good conductance. Maybe add a touch of solder for good measure.:beer
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
Just took it for the first drive.

It seems to have re-zeroed itself.
Now my idle is about 500 in gear, 1100 in park.

It climbs and falls smoothly.

Any ideas are welcome.

Thanks for the help!
 

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